There are several categories of blood cells. Erythrocyte or red blood cell (RBC) counts are for women 4.8 million cells/μl and men 5.4 million cells/μl. RBCs make up 93% of the solid element in blood and about 42% of blood volume. Platelets are 2 μm–3 μm in size. They represent 7% of the solid elements in blood and about 3% of the blood volume, corresponding to about 1.5 to 4×1011 cells per liter. There are 5 general types of white blood cells (WBCs) or leukocytes accounting for about 1.5 to 4×109 cells per liter. The WBCs comprise: 50–70% Neutrophils (12–15 μm in size); 2–4% Eosinophils (12–15 μm in size); 0.5–1% Basophils (9–10 μm in size); 20–40% Lymphocytes (25% B-cells and 75% T-cells) (8–10 μm in size); and 3–8% Monocytes (16–20 μmin size). They comprise 0.16% of the solid elements in the blood, and approximately 0.1% of the blood volume corresponding to around 4 to 12×109 per liter. A subject with an infection might have a WBC count as high as 25×109 per liter.
Platelets are the smallest cells in the blood and are important for releasing proteins into the blood that are involved in clotting. Patients with immune diseases that cause lower counts (such as cancer, leukemia and other chemotherapy patients) sometimes need platelet transfusions to prevent their counts from becoming too low. The platelet count in adults is normally between 140,000–440,000 cells/μl, and this number should not fall below 50,000 cells/μL because platelets play an integral role in blood clotting.
Blood separation techniques have traditionally employed discrete centrifugation processes. More particularly, a certain volume of blood is removed from a donor at a particular time. That volume of blood is then subjected to different levels of centrifugation to provide corresponding blood fractions for blood components such as plasma, platelets, red blood cells, and white blood cells. This process is discrete, rather than continuous, such that if more blood from the donor is to be processed, another volume is removed from the donor, and the process is repeated.
The steps in platelet collection are: collection of blood from donor: addition of anticoagulant; separation via centrifugation; return of red cells, leukocytes and plasma to the donor. A collection normally contains about 200–400 ml of plasma, which is reduced to avoid imcompatibility. This collection normally contains about 8 to 8.5×1010 platelets. A donor normally gives approximately 10% of his/her platelets with no loss in clotting ability, although a larger number of platelets could be separated from the blood. These platelets must be used within five days of collection.
Plateletpheresis, called apheresis, is a state of the art process by which platelets are separated [Haemonetics Component Collection System (CCS) and Multi Component System (Multi)(Haemonetics, Braintree, Mass.)]. This automated machine separates platelets from blood over a period of 1.5 to 2 hours (assuming 10% donation). This process is faster than traditional approaches and is completely automated and can be used for single or double platelet doses. Nevertheless, the process is slow relative to the patience of donors and is capable of improvement for the purity of the separated platelet fraction.
Other procedures are also time consuming, often taking several hours, particularly when unused blood fractions are to be returned to the donor. For example, platelet donation make take several hours, as whole blood is removed from the donor, fractionated through centrifugation to obtain the platelets, and the remaining blood components are then injected back into the donor. This centrifugation process is also comparatively harsh, also can result in damage to a proportion of the harvested cells, effectively reducing the usable yield of the blood fractions.
As a consequence, a need remains for a blood separation technique and apparatus which is continuous, has high throughput, provides time saving, and which causes negligible or minimal damage to the various blood components. In addition, such techniques should have further applicability to other biological or medical areas, such as for separations of cellular, viral, cell organelle, globular structures, colloidal suspensions, and other biological materials.